1. Field of the Invention
The present invention relates to a holographic radar having a plurality of reception antennas for receiving reflection waves of a radio wave transmitted from transmission antennas, and especially to such a holographic radar in which radar signals are acquired in a time-shared manner by the connections of the plurality of reception antennas switched in sequence to a receiver.
2. Description of the Related Art
An example of conventional holographic radar is disclosed in Japanese Patent Laid-open Publication No. Sho 63-256879. In that example, a single transmission antenna is provided to transmit a radio wave, with a plurality of reception antennas receiving reflection waves of the radio wave transmitted. The plurality of reception antennas are then connected via a switch to a single receiver. The plurality of reception antennas are switched in sequence for the connection to the receiver so that radar signals obtained for each reception antenna can be acquired in a time-shared manner.
Such a configuration enables the number of receiver for a plurality of reception antennas to be single and achieves a reduction in size and production cost of the device. It is to be noted that this example includes a reception antenna array consisting of multiple reception antennas, which are associated with a single receiver in groups of four antennas.
The radio wave used herein are in a high-frequency band of, for example, 76 GHz. For this reason, signals handled in the transmission line from the reception antennas to the receiver are signals belonging to the high-frequency band. The switch for switching such high-frequency signals typically has two or three throws.
Because of the above configuration, a plurality of switches are often required for the switching of four or more reception antennas. For example, multi-switching can be implemented through a combination of single pole double throw (SPDT) switches or single pole triple throw (SP3T) switches in a tournament form. Unit switches used therein can be planar circuit type high-frequency switches such as MMICs (microwave monolithic integrated circuits) or HICs (hybrid integrated circuits).
Use of switches in multi-stage connection, however, may cause a degradation in reception sensitivity since a signal will be attenuated every time it passes through each switch.